This is an auto-filed bug because media-video/mjpg-streamer calls cc directly.
The issue was originally discovered on amd64, but it may be reproducible on other arches as well.
If you think that a different summary clarifies the issue better, feel free to change it.
Attached build log and emerge --info.
If you think it doesn't make sense fix these type of issues, I'd like to point out that won't be possible use a different CC implementation (like clang) by setting the CC variable. So this issue has been reproduced by setting the CC variable to x86_64-pc-linux-gnu-gcc and by removing the /usr/bin/cc - /usr/bin/gcc binaries.
Created attachment 641182 [details]
build log and emerge --info
(In reply to Agostino Sarubbo from comment #1)
> Created attachment 641182 [details]
> build log and emerge --info
I'll try to work it out in upcomming days, also I'm covering this https://bugs.gentoo.org/665122 at the moment.
created new PR for thise and the other one: https://github.com/gentoo/gentoo/pull/16015